Misaligned Word Operand Read From Word Bus Example - Motorola MC68020 User Manual

WORD OPERAND (REGISTER)
15
OP2
D31
MSB
XXX
OP3
Figure 5-14. Misaligned Word Operand Read from Word Bus Example
Figures 5-15 and 5-16 show an example of a long-word transfer (write) to an odd address
in long-word-organized memory. In this example, a long-word access is attempted
beginning at the least significant byte of a long-word-organized memory. Only one byte
can be transferred in the first bus cycle. The second bus cycle then consists of a three-
byte access to a long-word boundary. Since the memory is long word organized, no
further bus cycles are necessary. Figure 5-17 shows the equivalent operation for a data
read cycle.
31
OP0
D31
MSB
XXX
OP1
Figure 5-15. Misaligned Long-Word Operand Write
5-18
0
OP3
DATA BUS D16
WORD MEMORY
LSB
OP2
XXX
LONG-WORD OPERAND
OP1 OP2
DATA BUS
LONG-WORD MEMORY
UMB LMB
XXX XXX
OP2 OP3
to Long-Word Port Example
M68020 USER'S MANUAL
MC68020/EC020
SIZ1 SIZ0 A2 A1 A0
1 0 0 0 1
0 1 0 1 0
0
OP3
D0
MC68020/EC020
LSB SIZ1 SIZ0 A2
OP0 0 0
XXX 1 1
MEMORY CONTROL
DSACK1 DSACK0
L H
L H
MEMORY CONTROL
A1 A0 DSACK1 DSACK0
0 1 1 L
1 0 0 L
MOTOROLA
L
L